The Lulu Chord is what I call a chord formed by playing a perfect fifth with a perfect fourth nestled inside it: for example, C-C#-F#-G. The outer notes, C-G are the fifth and the inner ones, C#-F#, are the fourth. This chord is non-diatonic and, as far as I know, unknown in tonal music. But it was very popular with the Second Viennese School and is a good thing to get a handle on if you're looking for some modernist vocabulary.
